A reagent preparation apparatus for preparing a reagent by mixing high concentration reagent and purified water is conventionally known. In a reagent preparation apparatus described in U.S. Patent Publication No. 2010/0055772, the purified water (RO water) produced in an RO (Reverse Osmosis) water producing section is stored in a RO water storage tank, and the RO water supplied from the RO water storage tank is used to prepare the reagent. In such reagent preparation apparatus, whether or not a predetermined amount of RO water is contained in the RO water storage tank is determined, and the RO water is produced by the RO water producing section if the predetermined amount of RO water is not stored in the RO water storage tank. The produced RO water is continuously supplied to the RO water storage tank if the RO water does not meet a predetermined amount, and the operation of the RO water production is stopped if the RO water of the RO water storage tank reached the predetermined amount.
In the reagent preparation apparatus described in U.S. Patent Publication No. 2010/0055772, the production of RO water is stopped when the RO water contained in the RO water storage tank reached the predetermined amount, and thereafter, the RO water is supplied from the RO water storage tank to prepare the reagent in a short time, and hence the RO water of the RO water storage tank again becomes lower than the predetermined amount and the production of the RO water is started. However, an operation for making ready to produce the RO water is necessary in the RO water producing section to resume the production of the RO water, and such operation requires time, and hence the production efficiency of the RO water lowers if production and stopping of the RO water are frequently repeated, and the reagent preparing process of the reagent preparing apparatus may not be efficiently carried out as a result. Furthermore, if production and stopping of the RO water are frequently repeated, the RO water production device connected to the reagent preparation apparatus may break down.